Semiconductor optical device

ABSTRACT

A semiconductor optical device includes an SOI substrate having a waveguide of silicon, and at least one gain region of a group III-V compound semiconductor having an optical gain bonded to the SOI substrate. The waveguide has a bent portion and multiple linear portions extending linearly and connected to each other through the bent portion. The gain region is disposed on each of the multiple linear portions.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2019-195931, filed on Oct. 29,2019, the entire contents of which are incorporated herein by reference.

FIELD

The present disclosure relates to a semiconductor optical device.

BACKGROUND

A technique for bonding gain regions formed of a group III-V compoundsemiconductor to a SOI (Silicon On Insulator) substrate (so-calledsilicon photonics) on which a waveguide is formed is known (for example,see “Ultra-low Noise Widely-Tunable Semiconductor Lasers FullyIntegrated on Silicon”, M. A. Tran et al., Compound Semiconductor Week2019, TuA3-1).

SUMMARY

A semiconductor optical device according to the present disclosureincludes an SOI substrate having a waveguide of silicon and at least onegain region of a group III-V compound semiconductor having an opticalgain bonded to the SOI substrate, wherein the waveguide has a bentportion and multiple linear portions extending linearly and connected toeach other through the bent portion, wherein the gain region is disposedon each of the multiple linear portions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view illustrating a semiconductor optical deviceaccording to a first embodiment.

FIG. 1B is a plan view of enlarging a vicinity of a gain region.

FIG. 2A to FIG. 2D are sectional views illustrating the semiconductoroptical device.

FIG. 3 is a plan view illustrating a semiconductor optical deviceaccording to a comparative example.

FIG. 4A is a plan view illustrating a semiconductor optical deviceaccording to a second embodiment.

FIG. 4B is a cross-sectional view illustrating the semiconductor opticaldevice.

DESCRIPTION OF EMBODIMENTS

To increase a monochromaticity of a laser beam by narrowing a spectrallinewidth, and to increase the output of the laser beam, a long gainregion is effective. The longer the gain region becomes, the larger asemiconductor optical device becomes.

[Description of Embodiments of the Disclosure]

Contents of embodiments of the present disclosure will be describedbelow.

One form of the present disclosure is a semiconductor optical device.(1) The semiconductor optical device comprises an SOI substrate having awaveguide of silicon and at least one gain region of a group III-Vcompound semiconductor having an optical gain bonded to the SOIsubstrate, wherein the waveguide has a bent portion and multiple linearportions extending linearly and connected to each other through the bentportion, wherein the gain region is disposed on each of the multiplelinear portions. It is possible to miniaturize the semiconductor opticaldevice by bending the waveguide. It is also possible to obtain goodcharacteristics such as narrow spectral line width and high output by aplurality of gain regions.

(2) The bending angle of the waveguide may be 90° or more. It ispossible to miniaturize the semiconductor optical device.(3) The semiconductor optical device may comprise a first insulatingfilm provided on both sides of the bent portion. The refractive indexdifference between the bent portion and the first insulating film islarge. And good effect of confinement light is achieved. Therefore, theloss of light in the bent portion is suppressed.(4) The semiconductor optical device may comprise a second insulatingfilm covering a side surface of the gain region, wherein a width of thegain region is larger than a width of the waveguide. Compared with thewaveguide, the light confinement of the gain region in the transversedirection is weak. Since the gain region is provided in the linearportion and does not bend, the loss of light is suppressed.(5) The radius of curvature of the bent portion may be 10 μm or more.Thus it is possible to miniaturize the semiconductor optical device.(6) The waveguide may include three or more of the linear portions, andthe gain region may be disposed on each of the three or more linearportions. Thus it is possible to obtain good characteristics, andminiaturize the semiconductor optical device.(7) The semiconductor optical device may comprise a first electrode anda second electrode provided on the SOI substrate, wherein the gainregion includes an n-type semiconductor layer, a core layer and a p-typesemiconductor layer that are stacked in this order from the SOIsubstrate, wherein the n-type semiconductor layer, the core layer, andthe p-type semiconductor layer are each formed of a group III-V compoundsemiconductor, wherein the first electrode is connected to the n-typesemiconductor layer, and the second electrode is connected to the p-typesemiconductor layer. It is possible to inject carriers into the corelayer by using the first electrode and the second electrode.(8) The at least one gain region comprises a plurality of gain regions,each of the gain regions having the core layer and the p-typesemiconductor layer, wherein the n-type semiconductor layer is shared bythe gain regions and electrically connects the gain regions, wherein thefirst electrode is provided on the n-type semiconductor layer, and has afirst connection portion and a first pad portion connected to the n-typesemiconductor layer, wherein the first connection portion is locatedbetween the gain regions, wherein the first pad portion is connected tothe first connection portion, wherein the first pad portion has a widthlarger than the first connection portion, wherein the second electrodehas a second connection portion and a second pad portion connected tothe p-type semiconductor layer, wherein the second connection portion isprovided on the p-type semiconductor layer of each of the plurality ofgain regions, wherein the second pad portion is connected to the secondconnection portion, and has a width larger than the second connectionportion. It is possible to inject carriers into the core layer by usingthe first electrode and the second electrode. Further, it is possible toreduce the electrical resistance by the first connecting portion and thesecond connecting portion.(9) The gain region may have a tapered portion located above thewaveguide. The efficiency of the optical coupling can be enhancedbetween the gain region and the waveguide.(10) The SOI substrate may be optically coupled to the waveguide andhave a resonator formed of silicon. It is possible to select awavelength of light by using the resonator.

Details of Embodiments of the Present Disclosure

A specific example of the semiconductor optical device according to theembodiment will be described below with reference to the drawings. Itshould be noted that the present invention is not limited to theseexamples, but is indicated by the claims, and it is intended to includeall modifications within the meaning and range equivalent to the claims.

First Embodiment

FIG. 1A is a plan view illustrating a semiconductor optical device 100according to the first embodiment. FIG. 1B is a plan view of enlargingthe vicinity of a gain region. FIG. 2A to FIG. 2D are cross-sectionalviews illustrating the semiconductor optical device 100.

As illustrated in FIG. 1A and FIG. 1B, the semiconductor optical device100 has a substrate 10 and a gain region 20. Semiconductor opticaldevice 100 is a tunable laser device of the hybrid type using a siliconphotonics. Three gain regions 20, two ring resonators 19, an electrode17, an electrode 30 and an electrode 32 are provided on the surface ofthe substrate 10. Surface of the semiconductor optical device 100 iscovered with an insulating film (not illustrated).

As illustrated in FIG. 2A, the gain region 20 is located above awaveguide 11. The gain region 20 comprises an n-type semiconductor layer22, a core layer 24 and a p-type semiconductor layer 26 which arestacked in this order. As illustrated in FIG. 2A to FIG. 2D, thesubstrate 10 is an SOI substrate comprising a substrate 12, a silicondioxide (SiO₂) layer 14, and a silicon (Si) layer 16 stacked in thisorder. The thickness of the SiO₂ layer 14 is, for example, 2 μm. Thethickness of the Si layer 16 is, for example, 220 nm. The waveguide 11and the ring resonator 19 illustrated in FIG. 1A are arranged in the Silayer 16 of the substrate 10. The end face of the substrate 10 is coatedwith an anti-reflection film to prevent reflection of light. Length L1in the X-axis direction of the semiconductor optical device 100 is, forexample, 1700 μm. Length L2 of the Y-axis direction of the semiconductoroptical device 100 is, for example, 600 μm.

As illustrated in FIG. 1A and FIG. 1B, the waveguide 11 has two bentportions 11 a and three linear portions 11 b. The bent portion 11 a has,for example, a semicircular arc shape. The bent portion 11 a may have ashape in which a clothoid curve or a raised cosine curve is combined.The linear portions 11 b are arranged one by one at both ends of thebent portion 11 a. Each linear portion 11 b extends in the X-axisdirection. The three linear portions 11 b are arranged in the Y-axisdirection, are separated from each other, and are connected to eachother via two bent portions 11 a. That is, in the region sandwichedbetween the two ring resonators 19 of the substrate 10, the waveguide 11bent 180° in the bent portion 11 a is disposed.

As illustrated in FIG. 1A, the linear portion 11 b located on the mostY-axis positive side of the three linear portions 11 b branches into twoin the vicinity of the end portion of the X-axis negative side of thesubstrate 10. In other words, two waveguides 11 are arranged. These twowaveguides 11 are optically coupled to the ring resonator 19 and reachthe end of the X-axis negative side of the substrate 10. The linearportion 11 b located on the Y-axis negative side of the three linearportions 11 b is branched into two near the end of the X-axis positiveside of the substrate 10. In other words, two waveguides 11 arearranged. These two waveguides 11 are optically coupled to the ringresonator 19 and reach the end of the X-axis positive side of thesubstrate 10. Both ends of the linear portion 11 b of the center of thethree linear portions 11 b are connected to the bent portion 11 a.

The gain region 20 is bonded on each of the three linear portions 11 b.The gain region 20 is not joined to the bent portion 11 a. One gainregion 20 overlaps one linear portion 11 b, and optically coupled witheach other. The gain region 20 has a linear shape extending in theX-axis direction, similarly to the linear portion 11 b. Length L3 in theX-axis direction of the gain region 20 is, for example, 800 μm.

The electrode 30 is an n-type ohmic electrode and has a pad 30 a andthree connection portions 30 b. The electrode 32 is a p-type ohmicelectrode and has a pad 32 a and three connection portions 32 b. The pad30 a is located on the Y-axis negative side of the three gain regions20. A connecting portion 30 b is electrically connected to the pad 30 a.The connecting portion 30 b is adjacent to the gain region 20, andspaced from the gain region 20. The connecting portion 30 b extends inthe X-axis direction. The pad 32 a is located on the Y-axis positiveside of the three gain regions 20. A connecting portion 32 b iselectrically connected to the pad 32 a. The connecting portion 32 b isarranged above the gain region 20. The connecting portion 32 b extendsin the X-axis direction.

The electrodes 30 are formed of metals such as gold, germanium or a Nialloy (AuGeNi). The electrodes 32 are, for example, laminates oftitanium, platinum and gold (Ti/Pt/Au). The thickness of the electrodes30 and 32 is, for example, 1 The width of each of the pad 30 a and thepad 32 a in the Y-axis direction is, for example, 100 μm or more. Thewidth of the connecting portion 30 b is, for example, 15 μm. An Auplating layer or the like may be provided on the electrodes 30 and 32.The electrode 17 is provided on top of the ring resonator 19 and isformed of a metal such as Ti, for example.

The gain region 20 and the n-type semiconductor layer 22 as illustratesin FIG. 1B has a tapered portion 21 and a tapered portion 23,respectively. The tapered portion 21 and the tapered portion 23 aretapered along the X-axis direction. The tapered portion 21 and thetapered portion 23 are located above the waveguide 11. The taperedportion 21 is provided at the end of the n-type semiconductor layer 22in the X-axis direction. The tapered portion 23 is above the taperedportion 21, and is provided at the end in the X-axis direction of thecore layer 24 and the p-type semiconductor layer 26 of the gain region20. The length of each of the tapered portion 21 and the tapered portion23 is, for example, 150 The width of each tip of the tapered portion 21and the tapered portion 23 is, for example, 0.4 The tapered portion 21and the tapered portion 23 are also provided on the end side of theX-axis negative side of the gain region 20.

FIG. 2A is a sectional view taken along a line A-A of FIG. 1A. FIG. 2Bis a cross-sectional view taken along a line B-B of FIG. 1A. FIG. 2C isa sectional view taken along a line C-C of FIG. 1A. FIG. 2D is across-sectional view taken along a line D-D of FIG. 1A.

As illustrated in FIG. 2A, the waveguide 11 and the groove 13 areprovided in the Si layer 16 of the substrate 10. The grooves 13 arelocated on both sides in the Y-axis direction of one waveguide 11. Thewidth W2 of the waveguide 11 and the width of the groove 13 is, forexample, 1 respectively. The SiO₂ layer 14 may be exposed in the groove13. The Si layer 16 may be a bottom surface of the groove 13. In theportion of the waveguide 11 overlapping the gain region 20, the sidesurface of the waveguide 11 is exposed to air.

An insulating film 34 is provided on the surface of the substrate 10. Asillustrated in FIG. 2B to FIG. 2D, the portion of the waveguide 11 thatdoes not overlap the gain region 20 is covered with the insulating film34. Between each of the waveguides 11 and between the waveguide 11 andthe Si layer 16 are embedded with The insulating film 34. As illustratedin FIG. 2B and FIG. 2C, the insulating film 34 is interposed between thewaveguide 11 and the pad 30 a. And the insulating film 34 is interposedbetween the waveguide 11 and the pad 32 a. The pad 30 a and the pad 32 aare not in contact with the waveguide 11. As illustrated in FIG. 2D, theside surface and the upper surface of the bent portion 11 a of thewaveguide 11 are covered with the insulating film 34. The bent portion11 a may not necessarily overlap with the pad, or may overlap with thepad.

As illustrated in FIG. 2A, the gain region 20 is located above thewaveguide 11. The gain region 20 has a structure in which the n-typesemiconductor layer 22, the core layer 24 and the p-type semiconductorlayer 26 stacked in this order. The width W1 in the Y-axis direction ofthe core layer 24 and the p-type semiconductor layer 26 in one gainregion 20 is, for example, 2 μm. The n-type semiconductor layer 22 isprovided over the three waveguides 11. The n-type semiconductor layer 22is shared by the three gain regions 20. The three gain regions 20 areelectrically connected by the n-type semiconductor layer 22. The sidesurfaces of the core layer 24 and the p-type semiconductor layer 26 arecovered with an insulating film 28. The distance D1 between theinsulating films 28 is, for example, 20 μm.

The n-type semiconductor layer 22 is formed of an n-type indiumphosphorus (n-InP) layer having a thickness of, for example, 0.3 μm. Thecore layer 24 includes a plurality of well layers and barrier layersformed of, for example, non-doped gallium indium arsenide phosphorus(i-GaInAsP). The core layer 24 has a multiple quantum-well (MQW: MultiQuantum Well) structure. The thickness of the core layer 24 is, forexample, 0.3 μm. The p-type semiconductor layer 26 is formed of, forexample, a p-InP layer having a thickness of 2 μm. The p-typesemiconducting layer 26 may further comprise a layer of p-type galliumindium arsenide (p-GalnAs) on top of p-InP. The insulating films 28 and34 are formed of an insulator such as SiO₂. The thickness of theinsulating film 28 is, for example, 0.5 μm. The thickness of theinsulating film 34 is, for example, 1.5 μm.

As illustrated in FIG. 2A, the pad 30 a and the connecting portion 30 bare provided on the surface of the n-type semiconductor layer 22. Thepad 30 a and the connecting portion 30 b are electrically connected tothe n-type semiconductor layer 22. As illustrated in FIG. 2B and FIG.2C, the pad 30 a and the pad 32 a are located on the surface of theinsulating film 34. The pads 30 a and the pad 32 a are spaced from thewaveguide 11. As illustrated in FIG. 2A, the connecting portion 32 b isprovided on the surface of the p-type semiconductor layer 26. Theconnecting portion 32 b is electrically connected to the p-typesemiconductor layer 26.

Each gain region 20 has a pin structure along the Z-axis direction. Byapplying a voltage to the electrode 30 and the electrode 32, carriersare injected into the core layers 24 of the three gain regions 20. Thecore layer 24 into which carriers are injected emits light. Lightpropagates through the waveguide 11 and enters the ring resonator 19.The ring resonator 19 reflects a portion of the light to the gain region20 side. The ring resonator 19 transmits a portion of the light. Lightcan be emitted from any one of the four waveguides 11 reaching the endof the substrate 10. Since the two ring resonators 19 have differentradii from each other. And the ring resonators 19 also have differentreflection spectrum. The wavelength at which the reflection peaks of thetwo ring resonators 19 coincide becomes the oscillation wavelength. Theelectrode 17 functions as a heater that generates heat upon input ofpower. By changing the temperature of the ring resonator 19 by theelectrode 17, the refractive index of the ring resonator 19 changes. Bythe change in the refractive index, the oscillation wavelength is madevariable within a range of, for example, 40 nm. The oscillationwavelength is, for example, 1550 nm±20 nm.

A method of manufacturing the semiconductor optical device 100 will bedescribed. The waveguide 11 and the ring resonator 19 are formed on thesurface of the wafer-like substrate 10. The substrate 10 may be providedwith an optical circuit such as a modulator. The p-type semiconductorlayer 26, the core layer 24, and the n-type semiconductor layer 22 areepitaxially grown in this order on a compound semiconductor wafer byusing a metalorganic vapor phase epitaxy (OMVPE: Organometallic VaporPhase Epitaxy) method or the like. The wafer is cut to form a pluralityof small pieces. Next, the small pieces are bonded to the substrate 10.As an example, the small pieces and the substrate 10 may be activated byirradiating the surface of the small pieces and the substrate 10 withplasma. By performing etching the small pieces, the gain region 20illustrated in FIG. 1A and FIG. 2A is formed. The substrate 10 is dicedto obtain a plurality of the semiconductor optical devices 100.

FIG. 3 is a plan view illustrating a semiconductor optical device 100Caccording to a comparative example. The waveguide 11 of thesemiconductor optical device 100C does not have a bent portion, extendsin the X-axis direction, and is optically coupled to the ring resonator19. The semiconductor optical device 100C has one gain region 40.

To narrow the spectral line width, and to obtain a high light output isrequired to increase the length of the gain region 40. For example, thelength L6 of the gain region 40 is made longer than the length L3 in thefirst embodiment in order to obtain a spectral line width and an opticaloutput equivalent to those of the three gain regions 20 in the firstembodiment in one gain region 40. The length L6 of the gain region 40is, for example, 2400 μm. For mounting a long gain region 40, the lengthL4 in the X-axis direction of the substrate 10 is larger than the lengthL1 of the first embodiment. The length L4 of the substrate 10 in theX-axis direction is, for example, 3300 μm. The length L5 of thesubstrate 10 in the Y-axis direction is, for example, 600 μm, which isthe same as L2. As described above, in the comparative example, thesemiconductor optical device 100C is increased in size. As a result, thecost is increased because the number of semiconductor optical elementsobtained from one wafer is reduced.

On the other hand, according to the first embodiment, the waveguide 11includes a bent portion 11 a and a linear portion 11 b. The gain region20 is joined to each of the three linear portions 11 b. Three gainregions 20 provide spectral line widths as narrow as one long gainregion 40 and high optical output. Since the waveguide 11 is bent, it ispossible to miniaturize the semiconductor optical device 100.

As illustrated in FIG. 1A and FIG. 1B, the waveguide 11 is folded by180° in the bent portion 11 a, and has a zigzag shape. Thus it ispossible to parallel the three linear portions 11 b on the substrate 10.Like the linear portion 11 b, three gain regions 20 are in parallel. Asa result, it is possible to effectively miniaturize the semiconductoroptical device 100. Specifically, the length L3 of one gain region 20 isabout ⅓ of the length L6 of the gain region 40. The length L1 in theX-axis direction of the semiconductor optical device 100 can be abouthalf of the length L4 of the comparative example. Therefore it ispossible to reduce the size of the semiconductor optical device 100 toabout 50% of the comparative example. Since it is possible to increasethe number of semiconductor optical elements 100 obtained from one waferto about 1.4 times, it is possible to reduce the cost.

The waveguide 11 is formed of Si. Grooves 13 are provided on both sidesof the portion of the waveguide 11 overlapping the gain region 20illustrated in FIG. 2A. Therefore both sides of the portion of thewaveguide 11 overlapping the gain region 20 are exposed to the air. Theportion of the waveguide 11 that does not overlap the gain region 20illustrated in FIG. 2B to FIG. 2D is covered with the insulating film34. The refractive index of Si is approximately 3.5. The refractiveindex of air is 1. The refractive index of the insulation film 34composed of SiO₂ is approximately 1.5. The refractive index differencebetween the waveguide 11 and the outside is large, and a lightconfinement of the waveguide 11 is enhanced. Therefore the loss of lightin the waveguide 11 including the bent portion 11 a is small.

The insulating film 34 provided on both sides of the bent portion 11 amay be formed of an insulator such as a SiN_(x) (x representscomposition) or a polymer in addition to SiO₂. To enhance the lightconfinement, the insulating film having a large difference of therefractive index with Si is preferable. Both sides of the bent portion11 a may be exposed to air. The groove 13 may be embedded in theinsulating film 34.

The radius of curvature of the bent portion 11 a is, for example, 50 μm.Therefore, it is possible to reduce the distance between the linearportions 11 b. Therefore it is possible to effectively miniaturize thesemiconductor optical device 100. The radius of curvature may be 50 μmor less, 30 μm or less, or 20 μm or less. The radius of curvature is 10μm or more. Miniaturization is possible by reducing the radius ofcurvature. Further, since the optical confinement of the waveguide 11 isenhanced, the loss of light, even if the radius of curvature is small,is suppressed.

On the other hand, the gain region 20 is formed of a group III-Vcompound semiconductor. The refractive index of the group III-V compoundsemiconductors is lower than that of Si. Thus the refractive indexdifference between the insulating film 28 on the side surface of thegain region 20 and the gain region 20 is smaller than the refractiveindex difference between the Si and the insulating film. Since thelateral light confinement is small, the loss of light is increased bybending the gain region 20 with a small radius of curvature. In thefirst embodiment, the gain region 20 is linear, and the waveguide 11 isbent. Thus, the loss of light is suppressed.

The core layer 24 and the p-type semiconductor layer 26 of the gainregion 20 have a high mesa structure with a narrow width. And, the sidesurface of the gain region 20 is exposed to air having a smallrefractive index. Thus, it is possible to suppress an increase in lossdue to bending at a small radius of curvature. However, the core layer24 of the high mesa structure processed to be as thin as the waveguide11 is apt to deteriorate with time when carrier injection is continuedfor a long period of time. The gain region 20 exposed to air is alsosusceptible to degradation over time from the sides of the MQW. Thewidth of the gain region 20 of the first embodiment is larger than thewaveguide 11. And, loss with time hardly occurs, because the sidesurface is covered with the insulating film 28. On the other hand, thetransverse light confinement is small and the loss due to bendingbecomes large. In order to suppress an increase in light loss, theradius of curvature becomes 200 μm or more, and miniaturization isdifficult. In the first embodiment, the strong waveguide 11 of the lightconfinement is bent, and a plurality of gain regions 20 have a linearshape. It is possible to achieve both suppression and miniaturization ofthe loss of light.

The gain region 20 has a pin structure having the n-type semiconductorlayer 22, the core layer 24 and the p-type semiconductor layer 26.Carriers are injected into the core layer 24 by applying voltages to theelectrode 30 and the electrode 32. The gain region 20 emits light byinjecting carriers.

The n-type semiconductor layer 22 is shared by the three gain regions20. The pad 30 a and the connecting portion 30 b of the electrode 30 areconnected to the n-type semiconductor layer 22. The electrode 32 has thepad 32 a and the connection portion 32 b. The connection portion 32 b islocated above each gain region 20. The connecting portion 32 b isconnected to the p-type semiconductor layer 26. By applying a voltagebetween the pad 30 a and the pad 32 a, carriers can be injected into theplurality of gain regions 20. As a result, the plurality of gain regions20 can emit light. In this example, it is possible to miniaturize thesemiconductor optical device 100, compared to the case of providing apair of pad 30 a and pad 32 a in each of the plurality of gain regions20.

The connection portion 30 b made of metal is provided between the gainregions 20, and the wide pad 30 a and the pad 32 a are provided on thesubstrate 10. Thus, the heat dissipation property is improved.Furthermore, the electrical resistance is also reduced. Incidentally,the pad 30 a of the electrode 30 illustrated in FIG. 2A is connected tothe n-type semiconductor layer 22. Therefore, even if the connectionportion 30 b extending adjacent to the gain region 20 is not provided,light emission of the gain region 20 is possible.

Light propagates in the X direction while spreading in a certain rangein the YZ plane and being distributed. Locating the metal electrodes 30and/or 32 inside the distribution of light increases the loss of light.Preferably, the electrode 30 and the electrode 32 are not in contactwith the waveguide 11. As illustrated in FIG. 2B and FIG. 2C, bycovering the waveguide 11 with the insulating film 34, contact betweenthe electrode 30 and the electrode 32 is prevented.

The gain region 20 has the tapered portion 21. The n-type semiconductorlayer 22 has the tapered portion 23. The width of the tip of the taperedportion 21 and the tapered portion 23 is 0.4 μm and narrow. Therefore,the efficiency of the optical coupling between the gain region 20 andthe waveguide 11 is improved to 90% or more. The gain region 20 may notnecessarily have the tapered portion 21. The n-type semiconductor layer22 may not necessarily have the tapered portion 23. Either one of thegain region 20 or the n-type semiconductor layer 22 may have the taperedportion. If the tapered portion is omitted, the efficiency of opticalcoupling is lowered. However, since the processing of the thin tip canbe omitted, the manufacturing becomes easy.

The number of the linear portions 11 b may be three, two, or four ormore. The number of gain regions 20 may be three, two, or four or more.It is possible to further miniaturize the semiconductor optical device100 by increasing the number of linear portions 11 b and/or the gainregion 20. However, even when the coupling efficiency is 90%, light lossmay occur in the coupling portion between the gain region 20 and thewaveguide 11. Therefore, as the number of the linear portions 11 b andthe gain regions 20 increases, the coupling portion increases and theloss of light also increases. To achieve both miniaturization andsuppression of loss of light, the number of the linear portion 11 b andthe gain region 20 is determined. The waveguide 11 is optically coupledwith the ring resonator 19. Therefore it is possible to select thewavelength in the miniaturized semiconductor optical device 100. Insteadof the ring resonator 19, a grating-type distributed reflector formed ofthe waveguide 11 made of Si may be provided for wavelength selection.Further, the two waveguides 11, instead of reaching the end of theX-axis positive side of the substrate 10, may be connected to a bentwaveguide for returning light such as a loop mirror waveguide.

Second Embodiment

FIG. 4A is a plan view illustrating a semiconductor optical device 200according to the second embodiment. FIG. 4B is a sectional viewillustrating the semiconductor optical device 200 and illustrating across section along the line E-E in FIG. 4A. Description of the sameconfiguration as that of the first embodiment is omitted.

As illustrated in FIG. 4A, the waveguide 11 has two bent portions 11 aand three linear portions 11 b. The bent portion 11 a corresponds to ¼of the arc. The linear portions 11 b are connected to both ends of thebent portion 11 a. The bending angle of the waveguide 11 is 90°. Two ofthe three linear portions 11 b extend in the X-axis direction. One endof the two linear portions 11 b is optically coupled with the ringresonator 19 in the vicinity of the X-axis negative end portion of thesubstrate 10, and the other end is connected to the bent portion 11 a.One of the three linear portions 11 b extends in the Y-axis direction.Both ends of the linear portion 11 b extending in the Y-axis directionis connected to the bent portion 11 a. The gain region 20 of the linearshape is joined to each of the three linear portions 11 b. The gainregion 20 is not bonded to the bent portion 11 a.

The electrode 30 and the electrode 32 are provided at a positionsurrounded by the three linear portions 11 b on the substrate 10illustrated in FIG. 4A. The electrode 32 illustrated in FIG. 4B islocated on the X-axis positive side than the electrode 30. The electrode32 is provided over the gain region 20 from above the insulating film34. According to the second embodiment, by bending the waveguide 11 by90°, the size of the semiconductor optical device 200 can be reduced toabout 50% of that of the comparative example, similarly to the firstembodiment.

The waveguide 11 of the first embodiment is bent by 180° and has a shapethat reciprocates in the X-axis direction. The waveguide 11 of thesecond embodiment has a U-shape that is bent by 90° and extends in theX-axis direction and the Y-axis direction. The waveguide 11 of thesecond embodiment may have other shapes. To miniaturize thesemiconductor optical element, the angle of bending of the waveguide 11is preferably 90° or more.

Although the embodiments of the present invention have been describedabove in detail, the present invention is not limited to the specificembodiments, and various modifications and variations are possiblewithin the scope of the gist of the present invention described in theclaims.

What is claimed is:
 1. A semiconductor optical device comprising: an SOIsubstrate having a waveguide of silicon; and at least one gain region ofa group III-V compound semiconductor having an optical gain bonded tothe SOI substrate, wherein the waveguide has a bent portion and multiplelinear portions extending linearly and connected to each other throughthe bent portion, wherein the gain region is disposed on each of themultiple linear portions.
 2. The semiconductor optical device of claim1, wherein a bending angle of the waveguide is 90 degrees or more. 3.The semiconductor optical device of claim 1, further comprising a firstinsulating film provided on both sides of the bent portion.
 4. Thesemiconductor optical device of claim 1, further comprising a secondinsulating film covering a side surface of the gain region, wherein awidth of the gain region is larger than a width of the waveguide.
 5. Thesemiconductor optical device of claim 1, wherein a radius of curvatureof the bent portion is 10 μm or more.
 6. The semiconductor opticaldevice of claim 1, wherein the waveguide includes three or more linearportions, wherein the gain region is disposed on each of the three ormore linear portions.
 7. The semiconductor optical device of claim 1,further comprising a first electrode and a second electrode provided onthe SOI substrate, wherein the gain region includes an n-typesemiconductor layer, a core layer and a p-type semiconductor layer thatare stacked in this order from the SOI substrate, wherein the n-typesemiconductor layer, the core layer, and the p-type semiconductor layerare each formed of a group III-V compound semiconductor, wherein thefirst electrode is connected to the n-type semiconductor layer, and thesecond electrode is connected to the p-type semiconductor layer.
 8. Thesemiconductor optical device of claim 7, wherein the at least one gainregion comprises a plurality of gain regions, each of the gain regionshaving the core layer and the p-type semiconductor layer, wherein then-type semiconductor layer is shared by the gain regions andelectrically connects the gain regions, wherein the first electrode isprovided on the n-type semiconductor layer, and has a first connectionportion and a first pad portion connected to the n-type semiconductorlayer, wherein the first connection portion is located between the gainregions, wherein the first pad portion is connected to the firstconnection portion, wherein the first pad portion has a width largerthan the first connection portion, wherein the second electrode has asecond connection portion and a second pad portion connected to thep-type semiconductor layer, wherein the second connection portion isprovided on the p-type semiconductor layer of each of the plurality ofgain regions, wherein the second pad portion is connected to the secondconnection portion, and has a width larger than the second connectionportion.
 9. The semiconductor optical device of claim 1, wherein thegain region has a tapered portion located on the waveguide.
 10. Thesemiconductor optical device of claim 1, wherein the SOI substrate isoptically coupled to the waveguide and has a resonator formed ofsilicon.